sensors
Article
An Optical Fibre Depth (Pressure) Sensor for Remote
Operated Vehicles in Underwater Applications
Dinesh Babu Duraibabu 1, *, Sven Poeggel 1 , Edin Omerdic 2 , Romano Capocci 2 , Elfed Lewis 1, *,
Thomas Newe 1,2 , Gabriel Leen 1 , Daniel Toal 2 and Gerard Dooly 2
1
2
*
Optical Fibre Sensors Research Centre, University of Limerick, Limerick V94 T9PX, Ireland;
[email protected] (S.P.); [email protected] (T.N.); [email protected] (G.L.)
Mobile & Marine Robotics Research Centre, University of Limerick, Limerick V94 T9PX, Ireland;
[email protected] (E.O.); [email protected] (R.C.); [email protected] (D.T.);
[email protected] (G.D.)
Correspondences: [email protected] (D.B.D.); [email protected] (E.L.); Tel.:+353-61-202-968
Academic Editor: Caterina Ciminelli
Received: 14 November 2016; Accepted: 15 February 2017; Published: 19 February 2017
Abstract: A miniature sensor for accurate measurement of pressure (depth) with temperature
compensation in the ocean environment is described. The sensor is based on an optical fibre
Extrinsic Fabry-Perot interferometer (EFPI) combined with a Fibre Bragg Grating (FBG). The
EFPI provides pressure measurements while the Fibre Bragg Grating (FBG) provides temperature
measurements. The sensor is mechanically robust, corrosion-resistant and suitable for use in
underwater applications. The combined pressure and temperature sensor system was mounted
on-board a mini remotely operated underwater vehicle (ROV) in order to monitor the pressure
changes at various depths. The reflected optical spectrum from the sensor was monitored online and
a pressure or temperature change caused a corresponding observable shift in the received optical
spectrum. The sensor exhibited excellent stability when measured over a 2 h period underwater
and its performance is compared with a commercially available reference sensor also mounted
on the ROV. The measurements illustrates that the EFPI/FBG sensor is more accurate for depth
measurements (depth of ~0.020 m).
Keywords: ROV; OFPTS; Pressure; sensors; ROV control; Fabry-Perot; Fibre Bragg Gratings
1. Introduction
Optical fibres have been widely used across a multitude of sensing applications [1]. Typical
measurands of optical fibres include pressure and temperature [2–6] with significant benefits
in the form of small size, immunity to electromagnetic interference, high sensitivity and low
cost [7]. Furthermore, many application areas have emerged, including biomedical, aerospace and
marine [8–11], where pressure/temperature sensors are required at a specific location. Optical fibre
based sensors can be utilized in either point or quasi-distributed configurations.
Fibre optic technology is often applied in the area of ocean science in the form of sensors and
communications and it is becoming increasingly essential to predict various ocean parameters in the
field of climate change, weather forecasts, energy supply, offshore mining and oil industries [12].
High resolution in-situ sensors which can be placed on buoys, moorings and Unmanned Underwater
Vehicles (UUVs) can be of major benefit in the monitoring of ocean parameters, e.g., ocean status
(pressure fluctuations), depth, temperature, salinity, etc. The environmental parameter changes
which need to be recorded are generally very small and existing sensors which have been mounted
on floats have to be frequently calibrated to achieve the required accuracy [13]. Optical fibre sensors
have several advantages over existing sensor technologies (quartz, inductive or capacitive) [14], e.g.,
include multiplexing capability, linearity, stability, range (1000 m) and accuracy (+/–2% of the depth).
Sensors 2017, 17, 406; doi:10.3390/s17020406
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A miniature sensor, based optical fibre technology, is described herein for high resolution
measurement of pressure (depth) with temperature compensation for the ocean environment. The
sensor, based on an optical fibre Fabry Perot interferometer (EFPI) approach, often used in a
number of sensing applications such as temperature, acoustic wave sensing, pressure and liquid level
sensing [15,16] will be used to measure pressure underwater. However, EFPI sensors often provide a
very high level of sensitivity of single particular physical parameters, but suffer from cross sensitivity,
e.g., temperature [17]. To overcome this, it is reported that temperature recorded in-situ can be
used to negate the cross-sensitivity issues. The sensor Fibre Bragg Gratings (FBG) is located at very
close proximity to the point of measurement. The developed sensor is mechanically robust and was
packaged, making it suitable for underwater use. Furthermore, the sensor diaphragm was designed
for the specific resolution and range needed for underwater applications. The combined pressure and
temperature sensor system was tested in its desired operating environment by mounting it on-board a
miniature remotely operated vehicle (ROV) and recording data whilst transitioning through varying
water depths. The reflected optical spectrum of the sensor was monitored online and a pressure
change caused a corresponding observable shift in the spectrum.
2. Sensor Theory and Design
Several pressure sensors based on EFPI sensors have been reported in recent years [17–20]. The
sensor described herein is fabricated entirely from silica glass and the EFPI sensor operation and
fabrication technique is discussed in the following section.
2.1. Operating Principle of EFPI and FBG
The schematic diagram of the EFPI sensor is shown in Figure 1 below. The incident light
propagating through the single mode (SM) fibre and is reflected at the glass/air interface of the SM
fibre, at the air/glass interface and at the glass/water interfaces of the diaphragm. The reflections
at these interfaces transmit back along the SM fibre and form an inference pattern. The phase
difference between the incident and the reflected light forms the interference which depends on the
cavity length. The relationship between the multiple interference and the back reflection spectrum is
expressed in Equation (1) below.
~0 + E
~1 + E
~2 )2
I = (E
I (λ) = E0 · E1 · cos
4πn0 L
λ
+ E0 · E2 · cos
4π (n0 L + n1 d)
λ
(1)
+ E1 · E2 · cos
4πdn1
λ
(2)
Where ’E0 ’ is the light reflected at the end-face of SM fibre, ’E1 ’ is the light reflected at inner side of
the diaphragm(MM), ’E2 ’ is the light reflected at the outer side of the diaphragm, ’L’ is the length
of the cavity, ’n0 ’, ’n1 ’ are the refractive indices of the air and diaphragm respectively, ’d’ thickness
of the diaphragm and ’λ’ the wavelength. The amplitude of the reflected light is determined by the
reflections of each interface. Cavity length changes with the deflection of the diaphragm when an
external pressure is applied. The change in cavity length ∆L due to the pressure difference may be
expressed as in Equation (3) [21].
∆L =
3(1 − v2 )
16Ed3
· a4 · ∆P = s p · ∆P
(3)
Where ’d’ is the diaphragm thickness and ’a’ radius, 0 υ0 Poisson ratio, ∆P is the change in pressure, ’s p ’
is the pressure sensitivity and E is the Young’s modulus. The deflection of the diaphragm depends
linearly on the pressure difference, when it is below 30% of the diaphragm thickness. Increasing the
diaphragm thickness results in the increase in the pressure range with decreased resolution.
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The light propagating through the SM FBG is reflected back if the optical wavelength is equal to
the Bragg wavelength λ B . The temperature sensitivity of the sensor occurs due of the effect on the
induced refractive index change and the thermal coefficient of the SM fibre. The Bragg wavelength
shifts λ B (∆T ) due to a change in temperature ∆T may be expressed as in Equation (4) where ’k’ and
’λ B ( T0 )’ are the temperature sensitivity and initial wavelength, respectively [22,23].
λ B (∆T ) = λ B ( T0 ) + k · ∆T
(4)
The compensation for the cross-sensitivity of thermal effects on the EFPI sensor is achieved by
using the matrix where the temperature sensitivity st is determined in advance [24].
"
# "
#"
#
∆λ B
0 k
∆P
=
(5)
∆L
s p st
∆T
Figure 1. Schematic diagram of the Extrinsic Fabry-Perot interferometer (EFPI) sensor.
2.2. Fabrication of Optical Fibre Pressure and Temperature Sensor (OFPTS)
The sensor fabrication process is shown schematically in Figure 2. The EFPI pressure sensor is
fabricated by splicing the polished capillary (outer diameter 200 µm) and a multimode (MM) fibre
(with a diameter of 200 µm), which forms a diaphragm. The single mode (SM) fibre with the Fibre
Bragg Gratings from FBGS Technologies GMBH is cleaved and inserted into the capillary.
The FBGs is 2 mm in length with a Bragg wavelength of 1554 nm and reflectivity of about 97%.
The FBG is completely enclosed inside the capillary and is then spliced to the capillary forming an air
cavity of ~20–30 µm using the Ericsson FSU 975 splicer. During splicing process the reflectivity of FBG
is reduced. The sensor head is cleaved and polished manually to a desired thickness (10–11 µm) and
then further etched chemically using hydrofluoric(HF) acid to reduce the thickness of the diaphragm
(3–4 µm). The complete fabrication process is monitored online in real time using a custom built
LabVIEWTM program which permits precise online monitoring of the diameter thickness during the
etching stage of the fabrication process. The schematic of the EFPI/FBG sensor and the reflection
spectrum is shown in Figure 3.
(a)
(b)
Figure 2. Cont.
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(c)
Figure 2. Sensor fabrication process: (a) The Capillary and the diaphragm (MM) fibre was polished
(b) The polished fibres are fused together (c) The single mode fiber with Fibre Bragg Grating (FBG) is
inserted into the capillary and fused with a air cavity.
(a)
(b)
Figure 3. (a) Schematic of the EFPI/FBG sensor; (b) Reflection spectrum of the sensor.
3. Setup for the Measurements
3.1. Optical System Setup
A schematic of the developed sensing systems and photograph are shown in Figure 4a and
Figure 4b respectively. The interrogation system consists of broadband light source(superluminescent
LED from Exalos EXS210069-01); a 3 dB coupler; an optical switch (Sercalo SW 1x4-9N); an optical
spectrum analyser (OSA) (Ibsen I-MON-512E). The broadband light source has a Gaussian spectral
shape with an optical power output upto 14.97 mW and with a bandwidth of 46.2 nm at 1554.2 nm is
used. The source was connected to a 3-dB optical coupler guiding the light to an optical switch which
has a 1 ms switching time. The light is reflected from the sensor and coupled back to the spectrometer
(Ibsen Photonics IMON 512E). The spectrometer has a resolution of 512 pixels with a bandwidth
from 1510–1595 nm. The optical switch was operated with a frequency of 20 Hz corresponding to
an interval of 5 frames/cycle and 2 cycles/sensor resulting in a 10 Hz sample rate for each sensor.
An Arduino board was used to control the optical switch with a switching period of 2 ms. The
interrogation time was adjusted depending on the sensor and the attenuator was used to balance the
optical power of the signal for the two sensors used.
3.2. On-Board Measurement System Setup
The measurement system mounted on board the ROV is shown photographically in Figure 5a.
The developed sensors can be used to accurately monitor the pressure in an ocean or similar
environment. However, if two pressure sensors are placed at opposite ends of an ROV the differential
reading from both sensors can be used in determining of the ROV depth accurately. The mount was
fixed on the ROV so that it can accomodate three sensors, two EFPI optical and one commercially
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available optical fibre pressure probe (OPSENS OPP-M) sensors as shown in Figure 5b. The OPSENS
sensor was placed exactly in the centre of the mount while the EFPI sensors were placed on the ends
on either side. The configuration provides critical information on the depth and the ROV’s stability
when moving underwater.
(a)
(b)
Figure 4. Optical System Setup: (a) Schematic of the optical system; (b) Optical System setup for using
two sensors.
(a)
(b)
Figure 5. Measurement setup (a) Sensors mounted on remotely operated vehicle (ROV), and
(b) Sensors placement on the mount.
4. Experimental Results
4.1. Calibration of Optical Pressure Sensors
The fabricated sensors were calibrated using a standard water column (Figure 6a) as well as a
commercial pressure chamber (Figure 7a) for use with pressures up to 10 bar (~101 m depth in water).
The pressure gauge (Badotherm BDT18-63MM) with accuracy of 1.6% full scale value (~163 cm H2 O)
and pressure range upto 10 bar was used as a reference.
The sensor was also placed in a standard water column and the response for the varying change
in pressure. The sensitivity of the sensor was determined as 10–20 nm/kPa with a resolution of 0.8 cm
H2 O. The sensor was placed inside the high pressure chamber and the pressure was raised gradually,
the FPI spectrum shift was observed based on the external pressure applied as shown in Figure 7b.
The pressure sensor was therefore further calibrated for a range extending from 0–90 m. The sensor
was calibrated for stability and noise for 1 h under 39 cm H2 O pressure as shown in Figure 6b with a
noise level less than ~0.5% of the full scale reading.
The OFPTS sensors exhibits a linear response as shown in Figure 8 when pressure is applied from
zero to 5 bar (5098.58 cm H2 O) with R2 = 0.99862. The cross sensitivity of the FBG due to pressure is
negligible as shown in Figure 7b under pressure since it is completely enclosed inside the capillary.
The cross sensitivity of the sensor (at 10 bar) due to water flow was estimated ~0.3% full scale value
(~30 cm H2 O) by a theoretical simulation model as shown in Figure 9b with flow of 6 knots(3 m/s).
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(a)
(b)
Figure 6. Measurement setup (a) Sensor calibration in a watercolumn, and (b) Calibration
measurement and the stability of the sensor for 1 h.
(a)
(b)
Figure 7. (a) Sensor calibration in the pressure chamber and (b) EFPI/FBG spectrum with initial and
under pressure.
Figure 8. Pressure response of EFPI/FBG sensor for 5 bar.
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(a)
(b)
Figure 9. (a) Theoretical Simualtion with no flow, and (b) Theoretical Simulation with flow of 6 knots
(3 m/s) at 10 bar pressure.
The OFPTS sensor was calibrated for temperature measurements using a temperature controller
block with a Peltier element designed for OFPTS (Figure 10a) and in a temperature chamber which
reaches 200 ◦ C. The copper block was mounted on top of the Peltier element and the temperature
was monitored using a thermocouple (Z2-K-1M). The reflection spectrum of the sensor is shown in
Figure 10b as the temperature was increased from room temperature to 130 ◦ C. The temperature
sensitivity of ~12.5 pm/K was determined with a resolution of 0.1 ◦ C.
(a)
(b)
Figure 10. (a) Sensor calibration in the temperature chamber; and (b) EFPI/FBG spectrum with initial
and under temperature.
4.2. Measurement on ROV
The optical fibre sensors of this investigation were packaged in a small tube and mounted on
the ROV as shown in Figure 5a. The ROV was controlled from an off-site location and was in
motion continuously inside the tank. The sensor was monitored online with a custom built LabVIEW
program which was used to capture and analyze all ROV data. The sensors response at varying
depths is shown in Figure 11. The OFPTS sensor was measured against the commercial sensor
mounted on the ROV and against the OPSENS probe. The OFPTS sensor was found to be more
sensitive than the electrical sensor mounted on the ROV and correlates with the OPSENS probe.
Figure 11 shows the response of the sensor for varying depths, which is split into three different parts
for analysis where section (A) represents the ROV moving slowly to the surface in the tank, section
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(B) represents the ROV moving slowly in the bottom of the tank and section (C) represents the ROV
moving rapidly to the surface and to the bottom of the tank.
Figure 11. Pressure (depth) response of the sensors mounted on the ROV where section (A) represents
the ROV moving slowly to the surface in the tank, section (B) represents the ROV moving slowly in
the bottom of the tank and section (C) represents the ROV moving rapidly to the surface and to the
bottom of the tank.
The Figure 12a shows the sensor response of the sensors when the ROV was moved up and down
in the tank. The OFPTS sensors 1 and 2 provide different values of the depth (orientation) while data
from the OPSENS stays in the middle. The difference in sensors 1 and 2 values is as expected as
the ROV was tilted in the horizontal while maintaining a constant depth. Figure 11(C) shows the
response of the sensors for repeatability when moved rapidly up and down in identical manoeuvre
in short intervals.
The Figure 13 shows the pressure response of the sensors when at the bottom of the tank when
moving continuously. The ROV was slightly tilted due to the umbilical cord during movement which
resulted in the small difference in depth between the sensors. The spikes in the sensor 2 was due to
the thrusters of the ROV.
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(a)
(b)
Figure 12. (a) Pressure (depth) response of the sensors mounted on the ROV from Figure 11 section
(A); and (b) Illustration of ROV orientation.
Figure 13. Pressure (depth) response of the sensors at the bottom of the tank when moving
continuously from Figure 11(B).
5. Conclusions
In this paper, the fabrication of two identical miniature optical fibre EFPI based sensors for
pressure (depth) measurements is discussed. The OFPTS sensors are based on EFPI and FBG was
tested on a small ROV in a lab based water tank for repeatability, resolution and accuracy against
electrical commercial sensors also mounted on the ROV. The commercial sensor on the ROV has
a depth resolution of only 4 cm which is inferior to the two OFPTS sensors. The investigation of
this paper has shown the two OFPTS sensors to have a resolution of 0.8 cm H2 O and an accuracy
of 2 cm H2 O. This work has demonstrated that the miniature OFPTS sensors can be utilised in the
marine environment as the full interrogation system can be completely housed in the marinised
pressure-resistant component of a typical ROV.
Acknowledgments: This work is supported by the Irish Research Council (EPSPG/2011/343), the Marine
Institute, Ireland, the European Regional Development Fund and Science Foundation Ireland (14/SP/2740 &
12/RC/2302).
Author Contributions: Dinesh Babu Duraibabu is the main corresponding author of this paper and contributed
to the refinement, fabrication and experimental work related to the sensor. Sven Poeggel contributed to the
fabrication and experimental work related to the sensor, as well as contributing to the authorship of this
paper. Edin Omerdic contributed to the experimental work related to ROV. Gerard Dooly and Romano Capocci
contributed to sensor experimental evaluations. Elfed Lewis (Director of the Optical Fibre Sensor Research
Centre) and Thomas Newe are principle investigators on this research project and contributed to authorship
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of this paper. Daniel Toal (Director of the Mobile and Marine Robotics Research Centre) and Gabriel Leen
contributed to the authorship of this paper.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
EFPI: Extrinsic Fabry-Perot Interferometer
FBG: Fibre Bragg Gratings
HF: HydroFluoric Acid
MM: Multi Mode
NI: National Instruments
OFPTS: Optical Fiber Pressure and Temperature Sensor
OSA: Optical Spectrum Analyzer
ROV: Remote Operated Vehicle
SM: Single Mode
UUV: Unmanned Underwater Vehicles
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